Bass is hard to get right, especially as the task is to translate the producers' projections onto our playback reality with a break in psycho-acoustic continuity. In other words, our 'ear-brain' knows our playback environment, but not the performance environment nor the producers' second guesses about our playback space. Over time, bass balance in production has gravitated toward a standard - in loudspeakers less so. There exist many products with strategically underdamped bass creating a big, loose hump in the upper bass. Think British monitors. Thiel's bass balance target was flat assuming only a floor under the speaker which neither added nor subtracted bass content. Any other assumptions are bound to be wrong because playback environments vary drastically and unpredictably.
Jim's design tools utilized free-air (hanging in a tree or later from a tight-rope) with the mic either 3 meters out or on the ground at various speaker heights. These free-field measurements were integrated with ground-plane measurements where the speaker was placed in the middle of a large, heavy truck grade asphalt parking lot (empty, after hours). Ground plane mic placed at 2, 3 and 4 meters out to average boundary conditions. Anechoic (free-air) measurements exhibit a -2dB shelf below 200Hz which comes up to flat in half-space - on a floor. The further room gain added by room reflections and resonances are matters of set-up and preference.
The SmartSubwoofers addressed those boundary effects with corrective circuitry.
There is a family of considerations for high frequency balance that I'll save for another time. Over the decades, criticism of Thiel speakers has been toward too-lean bass (rather than vice-versa), and popular opinion sometimes favors speakers with objectively heavy bass. Bass balance is a hard question because so much depends on the installation particulars and user preferences. My intent here is to say that Jim assumed nothing about structural reinforcement or subtraction of the bass in the playback space, leaving that to the end user. I haven't found a floor interface product that honors all the factors of that interaction. Jim's working assumption was to leave that set of interactions null. His speakers state his interpretation of correct on an imaginary, neutral floor.